Electrolytic production of tungsten and molybdenum



United States Patent 3,297,554 ELECTROLYTIC PRODUCTION OF TUNGSTEN AND MOLYBDENUM Harvey L. Slatin, New York, N.Y., assignor to Tirnax Associates, New York, N.Y., a partnership of New York No Drawing. Original application May 28, 1963, Ser. No. 283,697. Divided and this application June 23, 1966,

Ser. No. 567,029

3 Claims. (Cl. 20464) This is a division of my co-pending application Serial No. 283,697 filed May 28, 1963 and entitled Electrolytic Production of Tungsten and Molybdenum.

This invention relates to the production of tungsten and molybdenum directly from crude concentrates by fusion electrolysis.

In my Patent No. 2,960,451 entitled Electrolytic Production of Refractory Multivalent Metals a method for producing high purity tungsten or molybdenum is disclosed by electrolyzing tungsten anhydride (W0 or molybdic oxide dissolved in a fused bath containing alkaline earth metal halides, such as calcium chloride (CaCl and alkaline earth metal oxides, such as calcium oxide (CaO). When scheelite (CaWO or powellite (CaMoO is used in place of the tungsten anhydride or molybdic oxide as feed material, no calcium oxide is required since scheelite and powellite already contain the required calcium oxide.

In carrying out this latter embodiment of the process, the crude scheelite, powellite or processed molybdenum ores may be supplied directly to the electrolyte with a tungsten or molybdenum deposit of over 99% purity being obtained from the electrolyte. The metallic contaminants, such as iron or molybdenum which are usually in high concentration in the crude scheelite concentrate feed, appear in minor traces or in greatly reduced proportion in the deposit.

In continuous processing the concentration of calcium oxide in the electrolyte builds up as more and more scheelite or processed powellite concentrate is added to replenish the depleted tungsten or molybdenum. As the concentration of excess calcium oxide increases in the electrolyte, the quality of the electrodeposition process seems to be impaired so that the recovery efiiciency falls off and the purity of the deposit decreases. As described in my Patent No. 2,960,451 the concentration of the alkaline earth metal oxide may be in the range from below 5 weight percent to 40 and over with effective results. The preferred concentrations are in the order of -20% by weight and are related to the W0 concentration.

The primary object of this invention is to improve the process so that consistently high purity tungsten or molybdenum may be produced with high recovery efficiency in the fusion electrolysis of crude ore concentrate and calcium chloride electrolyte.

Another object of this invention is to provide a means for maintaining the electrolyte in such condition that the crystal size of the deposited tungsten or molybdenum remains large; the color of the deposit is silvery; the metal is substantially free of bronzes, lower oxides, carbides, borides, phosphides, silicides, sulfides, selenides, arsenides or the like.

These and other objects are realized by the invention by periodically converting the excess calcium oxide which builds up in the electrolyte to calcium chloride, which is the principal ingredient of the bath. This purpose may be attained in many ways, but it is preferable to bubble chlorine gas through the bath. By doing this periodically either before or after or during the addition of the ore, the electrolyte is sustained in such condition so as to maintain continued operation of the process.

-to 10% by weight solution calculated as tungsten.

3,297,554 Patented Jan. 10, 1957 ice The term high purity metal as used herein refers to metal whose purity lies above 99%.

The improvement embodied by the instant invention may better be understood by considering the following illustrations of my invention.

The electrolytic cell used in the following examples was made of graphite set in a welded shell crucible made of Inconel. The cell was 10 OD. and 8" I.D. and 14{ tall, inside height. The cell was not sealed airtight but the cathode electrodeposit was allowed to cool in a cylinder surrounding the cathode through which helium gas was passed, thus effectively isolating the cathode fromair and other gaseous contaminants. The electrolytic cell was made the anode to a depended cathode centrally located in the graphite crucible. The fused salt depth was about 1-0 inches with about 7" of cathode immersion. The cathode in these runs was an AGS-X grade graphite rod having an outside diameter of two inches and a length of three feet.

The cell assembly was placed in an electrically heated furnace which was thermostatically controlled. The direct current was supplied by a voltage adjustable rectifier.

The raw material feed was crude mill concentrate scheelite having the following petrographic analysis:

Vol. percent Pyrrohotite Garnet Actinolite 10-15 Epidote "J Chloride Scheelite Chemical analysis:

The solute was anhydrous calcium chloride weighing about 33 lbs. which was fused to a clear water-white melt. Sufiicient scheelite concentrate was added to provide a 5 This concentration appeared to give the best results. At higher concentrations scheelite crystallizes out and may interfere physically. The bath depth is about 10".

The operating procedure is as follows. A 2" OD. graphite rod, acting as cathode, is lowered into the molten bath foran immersion of seven inches. The current is adjusted to the selected cathode current density based on the 2" OD. X 7" immersion. A cathode that has been previously used in the electrolysis of tungsten is preferred, since in such use a thin film is produced which adheres tenaciously to the electrode and facilitates the adherence of subsequent electrodeposits.

The electrolysis is continued at the temperature set for 4 hours. Then the cathode with its electrodeposit and still under impressed DC. voltage is slowly withdrawn from the bath and retained in the cell. The fluid salts are allowed to drain back into the 'bath, During this time a clamshell-like steel cylinder surrounds the cathode through which helium or other inert gas may be passed to form an inert atmosphere around the deposit.

After the cathode has cooled sufiiciently to be handled, it is detached from the cathode holder and another electrode is put in its place. The electrolysis is recommenced.

The electrodeposit is stripped from the cathode and introduced into a container having a mechanical agitator such as a Lightning stirrer and a water inlet and weir overflow. In a few minutes the elect-rodeposited mass disintegra'tes as the soluble salts are washed away. Since the cell is not airtight, carbonates may form in the electrolyte. These are easily decomposed by HCl solution to facilitate the washing, Further aid to cleaning up the Duration of Run: 4 hrs. each (2 runs mixed). Temperature of Electrolyte: 1,000 O. Bath Conc.: W in CQClg (teed scheelite concentrate).

Voltage: 2.4 V.

Current: 150 amps. Cathode current density: 3.5 amps/in. initial.

Screen Analysis:

Size

a Chem. Analysis, W 7 95.5 Major Impurities Brick, Fe, C Brick, Fe, C

X-ray Impurities .0l W C .01 W C .3 scheelite. .2 scheclite.

metal deposit may comprise treating with caustic soda solution and the like. When the metal is saltfree by test for chloride ion, the deposited metal is transferred to a filter and air dried.

The'dried metal is silvery in color. It is classified as to particle size and analyzed for blending or other desired purpose.

The brick originated from improper handling of the furnace cover. The carbon appears through mechanical abrasion and the scheelite through mechanical entrainment. Much of the iron may be attributed to corrosion of the shell lining. With reasonable care and proper cell design, such extraneous impurities do not appear.

TABLE II Duration 0! Run: 4 hrs. each (2 runs mixed). Cathode: 2" Dia. grapite rod 7' immersion. Temperature of Electrolyte: 1.000 C.

Bath: 10' depth containing 10% by wt. W in 03.012. Anode: Cell crucible (graphite).

Voltage: 2.2 volts.

Current: 150 amperes.

Cathode current density: 3.5 amps/in. initial ealc.

Screen Analysis:

Slze +100 +150 200 +325 3.Z5 Wt. 3. 4 10. 9 15. 7 43. 5 26. 4 Chem. Analysis, Wt. Percent W 98. 8 90. 1 9S. 8 9S. 6 Principal Impurity Fe Fe Fe Fe X-ray Impurities None None None None None Each day or as required five pounds or more of schee- No scheelite or tungsten carbide was found in these lite concentrate is added to the electrolyte to replenish samples. Iron is heaviest in mesh, decreasing in the depleted tungsten.

I have found that by blowing dry the finer meshes to the lowest amount in -325 mesh.

chlorine gas through an inert tube made of quartz or The iron is not in excess of /2%. Also molybdenum was below 0.01%.

TABLE III Screen Analysis:

Wt. 12.1 19. 0 38.8 23. 5 Chem. Analysis W in Wt.

Percent 9 98. 5 98. 8 99. 0 99. 4

Fe Fe Fe Fe Not Taken TABLE IV Duration 01 Run: 4 hrs. each (2 runs mixed). Temperature of Electrolysis: 1,000 0. Bath Conc.: 10" depth, 2% by wt. calc. as W in CaCI Cathode: 2' dia. graphite, 7 immersion. Anode: Graphite cell pot. Cathode current density: 4.1 amps/in. cale. Volta e: 2.2 volts. Current: amperes.

Screen Analysis:

Size. +100 +150 +200 +325 --325 Wt. 1. 1 7.8 17. 2 55. 9 18. 0 Chem. Analysis W in Wt.

Percent 93. 4 9S. 4 98. 9 99. 0 98. 7 Principal Impurity.-- Brick, Fe Fe Fe Fe Fe X-ray Analysis Not Taken Although the scheelite concentrate used in the feed contained 4.5 wt. percent Fe and 3.5% M0, for some reason the cleaned electrodeposit from a regulated bath contained 6 material contained 48% Mo (98% CaM0O The conditions of electrolysis were identical to those run for tungsten. The results are shown in Table V.

TABLE V Screen Analysis:

Size

Wt. Principal Impurity, Fe

less than 0.25% Fe and less than .01% Mo. A possible explanation is that a CaCl electrolyte bearing about 10% by wt. of solute calculated as W salts out or precipitates CaMoO so that very little if any of the molybdenum is available for electrolysis. There is no question that the impurities are found in the sludge which forms in the bottom of the bath. The use of chlorine promotes the separation.

In a freshly prepared bath, the chlorine treatment speeds the time when the deposit fulfills required specifications. Otherwise the first deposits are usually discarded. The purity of the deposit improves with the age of the bath. However, after prolonged use the impurities build up in volume so that it becomes difl'icult not to physically entrain impurities despite the chlorine treatment. At this point it is practical to lade out the insolubles or start anew.

The preferred cathode material is graphite, but other inert electrical conductors may be used.

Although the large bulk of the market is for 98.8% W, where a product whose purity is over 99.99% W is required, the tungsten hereinabove described may be electorefined using the electrorefining method described in my Patent No. 2,960,451 or treated in the following way.

The electrodeposit stripped in accordance with the foregoing procedure is fed to a cell adequately covered, airtight and under inert gas pressure where it serves as anode. An inert cathode is immersed in the electrolyte for electrorefining of the tungsten. The preferred electrolyte is a 50-50 mol percent NaCl-KCl solvent containing 3-5 wt. percent W01 The WCl solute is easily generated within the cell by bubbling C1 through the bed of tungsten resting on the bottom of the cell. The anode current density is preferably below 10 amps/in. and 13 amps/in. is adequate. The cathode current density may be varied widely with good results from a few tenths of an ampere per sq. in. to over 35 amps/inF. The preferred cathode current density is in the range of 3-10 amps/m The temperature of electrolysis is 750- 800 C. The cathode and its electrodeposit are preferably cooled in an inert atmosphere. The adherent salts are readily washed away in water.

The tungsten metal crystals are macroscopic, silvery white, ductile and of extraordinarily high purity in excess of 99.99%.

The above mentioned apparatus was used to electrodeposit molybdenum from crude CaMoO This crude feed was provided by roasting molybdenite (M083) concentrates in the common commercial fashion. The raw The bolybdenum metal deposited was silvery bright in color and free from carbides, lower oxides, and the like. The iron content of the deposit was high due to the corrosion of the pot lining, the corrosion products falling into the bath. Normally, unless the bath is treated with chlorine gas prior to electrolysis, the first deposits are not as pure as subsequent deposits. The M0 in this example analyzed 97.0% for the reasons given.

In order to provide an electrolyte capable of producing molybdenum consistently having a purity in a bulk sample of 99.5% and selected crystals of 99.9%, chlorine gas is bubbled through the electrolyte of the foregoing runs set forth in Table V until free chlorine is detected in the efliuent gas stream. Also, as in connection with the tungsten, molybdenum may be used as an anode in an electrorefining process similar to the one used in connection with the electrorefining of tungsten to produce a pure molybdenum metal. Similarly, as in the case of tungsten, a molybdenum chloride is formed in the electrolyte which is preferably of about five weight percent.

The scope of the invention as described in the foregoing is set forth in the appended claims.

I claim:

1. A process for the electrorefiningof an impure refractory metal of the group tungsten and molybdenum comprising providing a fused electrolyte solvent containing sodium chloride and potassium chloride, providing a crude refractory metal of the group tungsten and molybdenum as anode and an inert cathode immersed in said electrolyte, bubbling chlorine gas through said eletrolyte so as to contact said crude refractory metal and thereby solubilizing said refractory metal as a chloride, electrolyzing said fused electrolyte thereby electrorefining said impure refractory metal.

2. A process for the electrorefining of an impure refractory metal of the group tungsten and molybdenum as set forth in claim 1 wherein said refractory metal is tungsten and said solubilized tungsten chloride is about 5 wt. percent in said electrolyte.

3. A process for the electrorefining of an impure refractory metal of the group tungsten and molybdenum as set forth in claim 1 wherein said refractory metal is molybdenum and said solubilized molybdic chloride is about 5 wt. percent in said electrolyte.

No references cited.

JOHN H. MACK, Primary Examiner.

H. S. WILLIAMS, Assistant Examiner. 

1. A PROCESS FOR THE ELECTROREFINING OF AN IMPURE REFRACTORY METAL OF THE GROUP TUNGSTEN AND MOLYBDENUM COMPRISING PROVIDING A FUSED ELECTROLYTE SOLVENT CONTAINING SODIUM CHLORIDE AND POTASSIUM CHLORIDE, PROVIDING A CRUDE REFRACTORY METAL OF THE GROUP TUNGSTEN AND MOLYBDENUM AS ANODE AND AN INERT CATHODE IMMERSED IN SAID ELECTROLYTE, BUBBLING CHLORINE GAS THROUGH SAID ELECTROLYTE SO AS TO CONTACT SAID CRUDE REFRACTORY METAL AND THEREBY SOLUBILIZING SAID REFRACTORY METAL AS A CHLORIDE, ELECTROLYZING SAID FUSED ELECTROLYTE THEREBY ELECTROREFINING SAID IMPURE REFRACTORY METAL. 